1. Field of the Invention
The present invention relates to a composition for hard-coating for protecting a soft substrate surface as a glass-like material and to a heat-shielding substance which shields heat rays by absorption.
2. Description of the Background Art
Plastic materials are frequently used in place of glass, taking advantage of their excellent characteristics such as transparency, light weight, resistance to impact, ease of processing, and the like. They cannot, however, take place of glass depending on the applications because of the reasons that the surfaces of the plastics are easily damaged, their resistance to chemicals is poor, and the like. In recent years, advances have been made in the technology of hard coatings applied to the surfaces of general-purpose engineering plastics, such as polymethylmethacrylate resins (hereinafter referred to as PMMA), polycarbonate resins (hereinafter referred to as PC), and the like, to provide major improvements in scratch resistance and chemical resistance, so that large amounts of these materials are now being used in the fields of optical parts, automotive parts, electrical appliances, building materials, and the like, where glass has conventionally been used.
Examples of materials used for hard-coating plastics include organic resin coating materials such as urethane resins, melamine resins, acrylic resins and the like, and inorganic coating materials such as organic silicates, silicone resins, metal oxides and the like. Of these, organic ultraviolet-curable acrylic resins and inorganic heat-curable silicone resins are used industrially.
Acrylic resins are superior from the aspects of economics and productivity, but are inferior to silicone resins in surface hardness and weather resistance. At the present time, silicone resins are the main group being used as hard-coating materials. A number of trials are being undertaken for providing silicone-type hard-coating compositions with functions such as anti-fogging, antistatistic, photosensitivity, semiconductance, antireflection, and dielectric characteristics, in addition to their inherent functions of protecting the surfaces of plastic materials. These additional functions can be obtained by dissolving or dispersing in the hard-coating composition chemicals which are capable of providing such functions.
A hard-coating composition of silicone resin type containing a photochromic compound has been proposed as a hard-coating material having light shielding characteristics. In this type of composition the degree of sunlight-shielding is controlled by the change in the color density proportional to the amount of light. The composition is expected to have application in sun glasses, window materials, and the like. However, its capability of shielding heat rays, particularly near-infrared rays, among sunlights is insufficient. In addition, photochromic compounds generally deteriorate by light and tend to lose their functions over time. They also change color through photodecomposition. Thus, their durability is inadequate.
Up to now, no hard-coating composition providing heat-shielding effects by the efficient absorption of near-infrared rays from sunlight has been developed.
On the other hand, a solar heat shielding film is known as a shielding material which shields heat rays from the sun by absorption or reflection. This type of film has a polyester film as a substrate to which an adhesive is applied to provide the function of preventing broken glass from flying and scattering, and on top of this adhesive layer, a colorant layer or a thin-film layer of aluminum or special metal is produced by vacuum evaporation to provide a heat shield function.
The colorant layer which is colored by a glazing dye, however, cannot absorb heat rays, i.e., near-infrared rays. An attempt to provide a sufficient heat shielding function to this type of film results in shutting out also visible lights, posing a problem of making the inside room dark.
Since a unit provided with a metal layer by vacuum evaporation exhibits high reflection of lights in near-infrared and visible region, it entails the environmental problem of a troublesome glare illuminating the surrounding area due to the reflected light, even though it has good heat-shielding characteristics. Another problem is its low percent transmission for visible light, which makes inside rooms dark. In addition, constructing multiple layers, including providing a metal layer by vacuum evaporation and forming a protective layer on top of the metal layer, requires much time and trouble with the attendant problem of high cost.
In Japanese Patent Laid-open (ko-kai) No. 3493/1990, a methods is disclosed for shielding solar heat rays by absorption by homogeneously dispersing a near-infrared absorbing composition directly into a plastic. However, for obtaining a thin film heat-shielding substance with a thickness of, e.g., about 100 .mu.m, the near-infrared absorbing composition must be used in a plastic in a large proportion. This can be achieved only by the sacrifice for visible light transmission, characteristics of films such as the film strength, the fluidity of the plastic required for the injection molding, and the like. Thus, no heat-shielding thin film with sufficient characteristics required in practical use can be obtained by this method.
Accordingly, an object of the present invention is to provide, with due consideration to the drawbacks of such conventional hard-coating compositions, a heat ray shielding substance having both functions of protecting a soft substrate surface and of shielding heat rays with long term stability and excellent efficiency and, at the same time, exhibiting high percent transmission for daylight (visible light) with low surface reflection, and a hard-coating composition for producing such a heat ray shielding substance.
Since the heat rays are mainly in the form of light from the near-infrared region, the inventors of the present invention studied incorporation of a near-infrared absorbing dye into a hard-coating composition in achieving the above objects. Near-infrared absorbing dyes which find application as an optical recording material using a semiconductor laser beam have been developed and various types of such dyes are available on the market. However, since these near-infrared absorbing dyes exhibit the absorption peak at a specific wave length in the near-infrared region, their effectiveness for shielding the entire heat rays is insufficient. Many of those having rather strong absorption in the visible range are intensely colored. In this respect, too, no suitable composition has yet been discovered. In addition, these dyes have insufficient heat resistance, their performance tends to deteriorate at curing temperatures, or their near-infrared absorption capability reduces over time. Furthermore, the poor solubility of the dyes causes them to deposit as time passes, making it impossible to obtain a transparent hard-coat layer. Thus, a satisfactory solution to the problems of the present invention was very difficult with these types of near-infrared absorbing dyes.
As a result of further painstaking research, the present inventors found that the object of the present invention could be achieved by the combined use of a derivative of thiourea and a copper salt of an organic acid, without directly using a near-infrared absorbing dye.